It is well known to exercise the body for purposes of muscle strength training by way of resistive training, that is where a selected muscle or group of muscles is exercised against a mechanical resistance. The training may be (a) isometric, in which the muscle length remains constant and the associated joint angle remains constant, (b) isotonic, in which the muscle shortens or lengthens at a constant force throughout the range of movement: the external force may change such that the muscle force remains constant as the mechanical advantage varies with joint angle, (c) DCER (Dynamic Constant External Resistance), in which the external force is constant and the muscle force may change with different joint angles, and (d) isokinetic, in which the joint angular velocity is constant and the muscle force varies with varying joint angle. It is therefore apparent that it is desirable to be able to change the force-displacement characteristics of a resistance training device.
It is also known to prescribe an exercise regime in terms of a number of repetitions of a force equal to a stated percentage of the maximum force which that subject can exert, which will obviously vary between subjects and over time.
There is therefore a need to provide exercise apparatus in which a resistive force can readily be varied for different subjects, different muscles within a given subject, or during the carrying out of a particular type of exercise. An object of the present invention is to provide a controllable load apparatus which would be useful in such an application.
Resistance training devices are known in the art which operate on the principle of compressing a gas, usually air. These devices require the provision of a compressed air supply which is used to precharge a cylinder to a starting pressure, and corresponding starting resistance, using a pressure regulator. Such a source of compressed air is not practical in a portable exerciser.
U.S. Pat. No. 4,880,230 describes a pneumatic exerciser comprising a double acting cylinder where the resistance is controlled by restricting the air flow between the chambers on either side of the piston. The resistive force of such a system is highly dependent on the velocity of activation, since it is based on flow restriction.
U.S. Pat. No. 4,257,583 describes a pneumatic exercise device which uses an air compressor to charge pressure reservoirs.